FLINT, Mich.— John McCain's unexpected selection of Alaska Gov. Sarah Palin as his running mate has confronted Barack Obama's campaign with a difficult challenge, as the Democrat seeks to undercut a new sense of excitement surrounding McCain without personally attacking a charismatic figure who is energizing a range of voters.

Aides said the Obama campaign would step up the use of female surrogates to counter the appeal of a woman on the Republican ticket and Obama primary rival Hillary Clinton hit the trail on his behalf in Florida on Monday. One adviser said the former first lady and Obama may have a joint appearance on-stage within the next two weeks.

But the campaign's key strategic imperative in the coming days is to counter the image of change agents that the McCain campaign has cultivated since it introduced the little-known Palin to the country as a maverick reformer from Alaska, an Obama aide said.

The Obama campaign has maintained its focus on the theme of change during the nearly two years since the Illinois senator kicked off his presidential run and his advisers believe that has been central to his success. So the campaign is determined not to permit the opposing camp to assume the mantle of change.

At an appearance in Flint, Obama mocked the idea that McCain and Palin would change the direction of the country, arguing that they would continue Bush administration policies on taxes, the economy, health care, education and Iraq.

The Obama campaign pushed back against a McCain television commercial released Monday that claimed Palin "stopped the Bridge to Nowhere," a reference to an infamous pork-barrel project that Sen. Ted Stevens (R-Alaska) inserted into a mammoth federal spending bill though Congress later removed the earmark following an uproar.

Obama campaign press secretary Bill Burton called the assertion in the commercial "a lie" and countered with news reports that Palin supported the bridge during her 2006 campaign for governor. The Obama press operation also e-mailed reporters a photo of Palin posing with a T-Shirt reading "Nowhere, Alaska 99901" during her campaign for governor. Palin canceled the bridge project after Congress rescinded the earmarked federal funding.

"You can't just make stuff up. You can't just recreate yourself," Obama said. "The American people aren't stupid. What they are looking for is somebody who's been consistently calling for change."

The lasting power of the surge the McCain campaign has experienced since he selected Palin as his running mate and she debuted on the national stage with her speech at the Republican convention last week remains unclear. Campaigns often experience a bounce in the polls after the sustained media attention of a nominating convention, and the Alaska governor remains a new and undefined figure to most Americans.

But as voters fill in their impressions of Palin, the Obama campaign and its Democratic allies plan to be actively involved in shaping perceptions.

An ABC News- Washington Post poll released Monday showed the presidential race in a statistical tie. The poll showed the biggest demographic shift over the period of the conventions was a swing of white women toward the McCain-Palin ticket, potentially assisted by the Alaska governor's "hockey mom" persona. White women moved from 50-42 for Obama to 53-41 percent for McCain—a 20 percentage-point swing.

Speaking to reporters at Obama headquarters in Chicago, campaign manager David Plouffe dismissed polls taken in the wake of Palin's nomination, saying commentators were "hyperventilating" over them and he expects the numbers to settle down in the coming days. He disputed the ABC News-Washington Post poll's finding on a shift among white women, saying the campaign's internal polls "are not seeing any movement like that."

The Obama campaign hopes to focus attention on some of Palin's more conservative positions. Among those are opposition to abortion rights even in the case of rape or incest and doubts she has previously expressed that human activity has contributed to global climate change.

Obama aides say they are hopeful that a focus on those issue differences will help win over moderate women voters—including Republican women— uncomfortable with a socially conservative policy agenda.

But it may be left to female surrogates to do much of the work in going after Palin. In addition to Clinton, Arizona Gov. Janet Napolitano, Kansas Gov. Kathleen Sebelius and Sen. Claire McCaskill (D-Mo.) have been prominent campaigners for Obama.

Women politicians are less vulnerable to charges of sexism, and the Obama campaign must confront a political environment in which many women feel the media and Obama campaign unfairly piled on Clinton during the primary campaign. Republican critics already have charged that the media has been insufficiently respectful to Palin during her national debut.

"There are risks in going after her because she is a woman, and that risk is amplified by the fact that Obama defeated a woman in the primary," said Democratic strategist Tad Devine, a senior adviser to the party's last two presidential nominees.

Devine noted that Obama is particularly vulnerable should his campaign tactics stir feelings of gender mistreatment because Democratic candidates typically depend on receiving outsized support from women while the Republican voter base is more male-dominated.

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